Cyclic adsorption or separation processes are well known and are typically used to separate a more adsorbable component from a less adsorbable component. The cyclic adsorption process employs a selective adsorbent to remove at least one component of a gas mixture and employs at least pressurization and depressurization steps, but more typically employs: (1) adsorption, (2) depressurization, (3) purge and, (4) pressurization steps. The feed gas containing the more readily adsorbable component and a less readily adsorbable component is passed through at least one adsorbent bed capable of selectively adsorbing the more readily adsorbable component at a predetermined (higher) adsorption pressure. The gas stream exiting the bed is concentrated in the less readily adsorbable component and is removed as product. When the bed becomes saturated with the readily adsorbable component, the bed is depressurized to a lower pressure for desorption of the readily adsorbable component so that this component can be selectively removed from the process.
Examples of such cyclic adsorption processes include but are not limited to pressure swing adsorption (PSA); vacuum swing adsorption (VSA); vacuum pressure swing adsorption (VPSA), all of which use a low pressure or a vacuum and a purge gas to regenerate the sorbent; temperature swing adsorption (TSA), which uses a thermal driving force such as a heated purge gas to desorb the impurities; and various variations of these processes. These adsorption processes are generally used to separate: oxygen or nitrogen from air; hydrocarbons and/or water vapor from feed air gases; hydrogen from carbon monoxide; carbon oxides from other gas mixtures; and other similar separations.
The cyclic adsorption processes useful herein will have at least one vessel containing at least one adsorbent bed therein (herein described as an “adsorber vessel” and an “adsorber bed”, respectively) and the adsorber bed can have one or multiple layers and various types of adsorbents. The processes can include the separation of gases for a wide variety of applications such as used to separate contaminates from end products in the air separation, refining, natural gas, chemical and petrochemical industries. Preferably, these processes are PSA, VSA, and VPSA air separation processes and most preferred is a VPSA air separation process to produce oxygen from air.
Adsorbents suitable for such adsorption or separation processes are well known and include molecular sieves, aluminas, silicas, zeolites, with and without binders, and the like. For the preferred PSA, VSA, and VPSA air separation processes, suitable adsorbents include, but are not limited to, A, X, and Y type zeolites, various ion exchanged forms of these zeolites, and silica-alumina, alumina, silica, titanium silicates, phosphates and mixtures thereof.
Cyclic adsorption plants for the separation of gases generally require industrial size compressors, also known as blowers, and associated equipment to pressurize and evacuate the adsorbent beds. Such compressors typically generate a wide range of frequency pulsations with associated levels of radiated noise generated by the pulsations. The noise is also generated by the pulsation induced vibrations of the equipment and by the gas flowing through the compressor and overall system. These pulsations are characterized by their frequency and wavelength. Frequency depends on the operating speed and specific design characteristics of the compressors, while wavelength (the speed of sound divided by the frequency) is a function of the pressure, temperature, and gas composition. The pulsations are also a function of the machine type; size; number of lobes or blades or other active components; and, most importantly, the speed of operation. For each compressor type, there is a speed range of operation that generates pulsations at various frequencies.
Conventional cyclic adsorption plants, such as a VPSA plant, most often use rotary type positive displacement compressors which are known to generate pulsation frequencies in the lower range such as about 20 Hertz (Hz) to about 450. Higher frequency pulsations are also possible depending on the specific design characteristics and operational parameters of the compressor. Dynamic displacement compressors, such as centrifugal or axial compressors, have been proposed for use in such applications, based on new designs and improved capability. These compressors also generate pulsations, but often operate at different speeds causing different ranges of frequency pulsations.
Such pulsations are considered undesirable since the noise levels (sound waves) generated or otherwise associated with these pulsations result in undesirable conditions. High noise levels are considered a safety concern and acceptable levels are often governed by environmental regulations or ordinances where the cyclic adsorption plants are located. While plant noise levels can easily range from 170 to 180 A-weighted decibels (dBA), such regulations or ordinances typically require the sound levels to be less than 90 dBA and often less than 85 dBA over a 24 hour period. Even within these environmental requirements, plant workers must still wear protective equipment to protect their hearing and local residents remain subjected to nuisance levels of ambient noise at or near the plant fence line. Thus, noise attenuation is often required to meet environmental and regulatory requirements.
In addition, certain frequency pulsations, generally in the low to medium range, cause vibrations that can fatigue and otherwise damage pipes, pipe couplings, adsorber vessels, adsorber beds, adsorbent materials, valves, and associated equipment thereby requiring increased maintenance and equipment costs. These low to medium range frequencies generate harmonic frequencies at multiples of the fundamental frequency produced, thereby contributing to the total power (dBA level) of the noise and the magnitude of the vibrational effects. For this reason, cyclic adsorption plants often require special piping systems and equipment designs to address these pulsation effects resulting in adding costs and additional operating considerations.
To reduce pulsations and the associated radiated noise and vibrational effects from the operation of the plants, conventional cyclic adsorption plants employ sound attenuation equipment known as “silencers”. One or more of these silencers are placed at the feed and/or discharge of each compressor/blower to reduce the pulsation effects. For example, U.S. Pat. No. 7,819,223 illustrates a conventional VPSA air separation plant with the required silencer to decrease noise. Such silencers are commercially available and include a cylindrical steel-shell type having multiple chambers, Helmholtz resonator type pulsation dampeners, and partially buried chambers with impedance tubes and baffles to provide noise attenuation. Silencers taught for use in cyclic adsorption plants are also described by U.S. Pat. No. 7,695,553. Such silencers attenuate low frequency pulsations using reactive components/chambers and attenuate medium and high frequency pulsations using sound absorbing components/chambers. The reactive components or chambers primarily provide peak noise reduction in the frequency range of less than 250 Hz (low range) and the absorptive components or chambers provides peak noise reduction in the frequency range from about 250 to 500 Hz (medium range). One drawback with the use of reactive type silencers is that they cause a considerable pressure drop associated with the cross sectional discontinuities required for low frequency noise attention. These pressure drops must be addressed in the process design resulting in additional power consumption and lost process efficiencies. For example, in a typical VPSA plant the pressure drop resulting from the reactive components in the feed inlet and vacuum discharge silencers can be as high as 0.25 psid per silencer. This pressure drop must be overcome by operating the compressors to raise the pressure ratios required for operating at the necessary process conditions. Typically, the pressure drop caused by the reactive type silencers results in approximately 5 percent higher power consumption in a typical VPSA process.
Further, as cyclic adsorption plants become larger, these silencers must also become larger in both length and diameter, to provide the necessary sound attenuation. Large silencers significantly increase plant costs, are more prone to vibrational and mechanical failure, and increase the overall footprint of the plant, requiring additional land property that is not always available. Occasionally, silencers for these larger plants will still not meet the requirements for attenuated pulsations and associated noise, which results in requiring additional noise abatement techniques, such as the use of buildings, noise enclosures, and acoustic insulation systems.
Thus, the costs of manufacturing, installing, and maintaining silencers and the operation of the plant with such silencers, with the resulting pressure drop, become a significant capital and operating consideration and can add significantly to the cost of designing and operation such plants. By removing or reducing the size of such silencers, significant capital and operating savings can be achieved.
It has now been found that the compressors used in cyclic adsorption processes can be operated under conditions which meet the operational requirements of the cyclic adsorption process, but can be controlled to operate under conditions that eliminate low and, preferably, low and medium frequency pulsations. According to this invention, the compressor can be operated at predetermined speeds which do not generate low range frequency pulsations eliminating the need for reactive type silencers and reducing the needs for absorptive type silencers. For example, the passive silencers (e.g., non-reactive absorptive type) may be smaller or less rigorously constructed, and therefore require less expensive materials. In a preferred mode of operation, using the preferred centrifugal compressor, low and medium range frequency pulsations can be eliminated and the plant is operated in the absence of either a reactive or passive (sound absorbing) type silencers.